As heat insulating materials having lower thermal conductivity and excellent heat insulating performance, there have been known low-thermal-conductive heat-insulating materials obtained by mixing ultrafine granular anhydrous silica which is a low thermal conductive material (for example, Aerosil (trade name) manufactured by Nippon Aerosil Co., Ltd), a ceramic fiber and further a radiation absorption scattering material comprising particles of silicon nitride, titanium oxide, zirconium oxide or the like, and press molding the resulting mixture, followed by machining (see the following patent documents 1 to 6).
Patent Document 1: JP 07-267756 A
Patent Document 2: JP 10-509940 A
Patent Document 3: JP 10-509941 A
Patent Document 4: JP 11-513349 A
Patent Document 5: JP 11-514959 A.
Patent Document 6: JP 2000-506570 A
The above-mentioned ultrafine granular anhydrous silica or supercritical dry silica is fine particles having a diameter of several nanometers to tens of nanometers, and a low thermal conductive material having a thermal conductivity at ordinary temperature (25° C.) of about 0.01 W/(m·k). However, the ultrafine granular anhydrous silica or supercritical dry silica associates by cohesive forces to form secondary particles, because they are fine particles having a diameter of several nanometers to several tens of nanometers. As schematically shown in FIGS. 8(A) and 8(B), the secondary particles 10 are dispersed among inorganic fibers 1F such as ceramic fibers or glass fibers. Accordingly, interfiber thermal conduction occurs at portions where the inorganic fibers 1F are entangled, which causes the low thermal conductivity of the ultrafine granular anhydrous silica or supercritical dry silica to be largely impaired. For example, glass fiber has a thermal conductivity of about 0.1 W/(m·k), and the heat insulating performance of the whole heat insulating material greatly depends on the thermal conductivity of the glass fiber.
Further, a radiation absorption scattering material 1a is effective for reducing thermal conduction in a high-temperature region exceeding 200° C. However, solid conduction occurs between the adjacent radiation absorption scattering materials, and particularly, thermal conduction in a low-temperature region of 200° C. or lower increases. Still further, solid conduction also occurs between the radiation absorption scattering material 1a and the inorganic fiber 1F.
Furthermore, a kneaded product of the inorganic fiber 1F, the ultrafine granular anhydrous silica or supercritical dry silica and further the radiation absorption scattering material 1a is press molded using no binder, attaching importance to the heat insulating performance. This causes insufficient mechanical strength, so that the heat insulating material is cracked or chipping of cut edges is liable to occur. It has therefore the disadvantage of being poor in handling ability or processability. Further, the secondary particles 10 of the ultrafine granular anhydrous silica or supercritical dry silica merely get into among the fibers, so that adhesion force between the secondary particles 10 and the inorganic fiber 1F is low. Accordingly, the secondary particles 10 are eliminated (powder omission) to contaminate the outside. There is therefore also the problem that it is difficult to use for applications requiring cleanness, such as a semiconductor manufacturing apparatus.